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Mr. Robert Doss, P.E. <br /> April 96, 199'1 <br /> Page Two <br /> establishing soil cleanup levels that are protective of <br /> groundwater. Following are the specific areas requiring <br /> additional characterization: <br /> 1. The highest levels of gasoline and diesel constituents have <br /> been detected in the shallow (Zone A) wells MW-4 , MW-8, <br /> MW-9, and MW-10. Additional characterization work is <br /> required to delineate the vertical and horizontal extent of <br /> the fuel plume(s) . This additional work should also serve <br /> to characterize the elevated cyanide levels at MW-9 . <br /> 2 . PNA's appear to be fairly widespread throughout the site at <br /> levels significantly above background levels. For Zone A, <br /> MW-2 is located furthest in the upgradient direction and <br /> thus may serve as a representative background well . <br /> Elevated Polynuclear Aromatics (PNA) levels were primarily <br /> detected in the same Zone A wells listed in item #1 above <br /> with the addition of MW-11 and MW-12 located immediately <br /> offsite in the downgradient direction. Additional <br /> characterization work is necessary in Zone A to establish <br /> background concentrations and to determine the horizontal <br /> extent of PNA migration offsite. In the lower shallow zone <br /> (Zone B) , all three Zone B wells have detected elevated <br /> concentrations of PNA's. Further characterization work is <br /> required to establish background concentrations for Zone B <br /> and to delineate the extent of contamination above <br /> background. Similar characterization work is required for <br /> the deep zone (Zone C) . Zone C is currently monitored by a <br /> single well (MW-13) which has detected the highest levels of <br /> naphthalene for the site. Information provided in previous <br /> reports indicate that groundwater samples taken from <br /> upgradient deep wells within one mile from the site have <br /> never detected naphthalene. Therefore, it is unacceptable <br /> to dismiss significant contaminants levels found in MW-13 by <br /> implying they may be comparable to background levels. The <br /> vertical and horizontal extent of contamination in Zone C is <br /> unknown. <br /> 3 . Detection levels for carcinogenic PNA's must be set <br /> appropriately to detect levels which may be above the <br /> established health based carcinogenic risk level of <br /> 0. 0028 ug/l (U. S. Environmental Protection Agency (EPA) <br /> National Ambient water Quality Criterion) . It is <br /> unacceptable to eliminate a PNA from the chemicals of <br /> concern based on non-detection via an insensitive method. <br /> Samples must be prepared for analysis by a clean-up <br /> procedure approved by Environmental Protection <br />